1. Field of the Invention
The present invention relates to an anticorrosion sleeve that prevents an exposed portion of a metal pipe in water mains from corroding when an opening is drilled in the metal pipe.
2. Description of the Related Art
When a branch pipe is connected to a water main pipe in a conventional method, an opening is drilled in the water main pipe and a sleeve is fitted to the opening to prevent corrosion on the wall of the opening. Since the sleeve is typically metallic, a rust deposit is caused if gaps take place between the metal sleeve and the sleeve passage or between the metal sleeve and the wall of the opening of the main water pipe. A water-tight sleeve made of rubber is also known. If the rubber sleeve is forced into the opening too strongly, the rubber may be buckled, and water may be leaked. The opening of the pipe is typically not smooth and can have a sharp edge. When engaged with such a sharp edge, the rubber sleeve is likely to be cut and no sufficient anticorrosion effect is expected.
The inventors of this invention disclose a dual-structured anticorrosion sleeve having a hybrid sleeve with a resin and a metal in Japanese Utility Model Open Gazette No. 7-23892 and Japanese Patent Open Gazette No. 2001-304487.
In accordance with Japanese Utility Model Open Gazette No. 7-23892, the metal sleeve provides rigidity while the resin sleeve provides tightness. However, the metal sleeve has a complex cross-sectional structure to assure tightness between the resin sleeve and the metal sleeve. This structure leads to an increase in manufacturing cost and a large force must be applied to expand the diameter of the metal sleeve and to pull a diameter expansion device out of the assembly.
An anticorrosion sleeve disclosed in Japanese Patent Open Gazette No. 2001-304487 has been developed to overcome the drawback of the above-referenced sleeve. The disclosed sleeve includes a metal sleeve and a resin sleeve. The metal sleeve includes a step on the outer circumference between a large-diameter portion and a small-diameter portion. In accordance with the structure of the metal sleeve, the resin sleeve includes a step on the inner circumference between a small-diameter portion and a large-diameter portion along the direction of insertion. The step of the metal sleeve is thus engaged with the step of the resin sleeve. With the steps used as a guide, the metal sleeve is inserted into the resin sleeve without using any particular fitting tool. The resin sleeve is expanded in diameter by simply inserting further the metal sleeve and the sleeve is thus mounted. A significant force is required to force the metal sleeve into the resin sleeve to expand the diameter of the resin sleeve. To assure reliable tightness with the opening of the water main pipe, a predetermined criterion in the expansion must be set beforehand. Such a predetermined criterion is not applicable to a large opening and the degree of diameter expansion of the resin sleeve is typically small. Tightness on the large-diameter portion becomes insufficient.